Rachiplusia

Members of Rachiplusia are distinguished from other Plusiinae genera by genitalic characters established through taxonomic revision. Larvae of R. nu can be separated from the similar Chrysodeixis includens (soybean looper) by spinneret shape, though this requires microscopic examination. Field identification of R. nu larvae relies on coloration: older larvae exhibit a darker, smoky-blue cast compared to the bright yellow-green of C. includens, and possess distinct patches of tiny black asperites on the thoracic venters not apparent in the latter species.

Agricultural landscapes, particularly soybean and sunflower fields. Dry conditions favor population increases, while moist conditions promote pathogenic fungi that suppress larval populations. Early season populations often establish in alfalfa and flax before moving to soybean and sunflower.

South America (Argentina, Bolivia, Brazil, Chile, Paraguay, Peru, Uruguay) for R. nu; North and Central America for R. ou. R. nu is the most widely distributed South American species in the genus.

Infestations in soybean tend to occur during reproductive stages of plant growth. Activity patterns align with crop phenology, with early season establishment in alternate hosts followed by movement to soybean and sunflower.

R. nu larvae feed on soybean (Glycine max), sunflower, alfalfa, and flax. Young larvae consume only the lower epidermis between leaf veins, creating "window paning" damage. Older larvae consume entire tissue layers while avoiding vascular tissue, resulting in skeletonized foliage. A single larva can consume more than 100 cm² of soybean foliage.

• Glycine max - food plantprimary host, soybean
• Helianthus annuus - food plantsunflower, heavily attacked in drier western regions
• Medicago sativa - food plantalfalfa, early season host
• Linum usitatissimum - food plantflax, early season host

Complete metamorphosis: egg, larva, pupa, adult. Eggs are laid primarily on undersides of leaves. Neonate larvae begin feeding on lower leaf surfaces. Larvae pass through multiple instars, with older larvae causing the most significant damage. Pupation occurs in soil or leaf litter. Multiple generations per year are possible in favorable conditions.

Larvae exhibit characteristic looping locomotion due to reduced proleg number (three pairs), typical of Plusiinae. Young larvae feed gregariously or in close proximity to egg mass sites before dispersing as they grow. Adults are nocturnal and have been observed hovering at flowers during dusk without landing.

Herbivore and agricultural pest. Historically a secondary pest of soybean, R. nu has become a primary pest due to evolution of resistance to Cry1Ac Bt toxin. Larvae serve as hosts for pathogenic fungi that regulate populations under moist conditions. The species is a target for biological control using baculoviruses such as Autographa californica multiple nucleopolyhedrovirus (AcMNPV), registered in Brazil in 2023.

Significant economic pest of soybean and sunflower in South America, particularly Argentina and Brazil. Yield losses occur when defoliation exceeds approximately 15% during reproductive plant stages. Management is complicated by differing pesticide susceptibilities between R. nu and the co-occurring Chrysodeixis includens, leading to control failures and increased pesticide applications when misidentification occurs. Resistance to Cry1Ac Bt toxin has been documented in Brazilian populations. Subject of resistance monitoring using diagnostic concentrations of AcMNPV.

• Chrysodeixis includensLarvae nearly identical in appearance and share soybean as host; distinguished by spinneret shape, larval coloration (bright yellow-green vs. smoky-blue in older R. nu), and thoracic ventral asperites; differs in pesticide susceptibility and geographic distribution
• Pseudoplusia includensFormer name for C. includens; taxonomic synonymy now resolved under Chrysodeixis
• Anticarsia gemmatalisCo-occurring noctuid defoliator of soybean in South America; distinguished by larval morphology and behavior—A. gemmatalis has four pairs of prolegs and does not exhibit looping locomotion
• Spodoptera spp.Armyworm species co-occurring in soybean; distinguished by larval morphology including number and arrangement of prolegs, pinaculae, and head capsule markings

• agricultural pest
• soybean pest
• sunflower pest
• Plusiinae
• Noctuidae
• South America
• Bt resistance
• biological control target
• looping caterpillar

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Noctuidae | Beetles In The Bush | Page 3
• armyworms | Beetles In The Bush
• Baseline Susceptibility and Diagnostic Concentration of AcMNPV in Rachiplusia nu (Lepidoptera: Noctuidae) in Brazil and Cross-resistance to Cry1Ac.